When a well is drilled in a subsea environment the ability to repair, remove, or maintain the industry standard well control device, the Blow Out Preventer, BOP, is challenged due to the fact that it is on the seafloor. As mankind continues to drill wells in ever deeper water depths the BOP system may be miles below the sea surface.
The subsea BOP currently used by the industry is further compromised or challenged by the large riser pipe that can be several feet in diameter, miles long, and have a weight of millions of pounds being mounted on top of the BOP stack and proceeding to the surface. This large riser is used to allow return fluids from drilling the well to flow from the subsea well head to the surface through the BOP. If the riser pipe fails then there is no path to get drill pipe or kill fluids into the subsea well. If the riser pipe fails, and the BOPs fail during a blow out, like it did in the 2010 Gulf of Mexico blowout, then there is no method to get drill pipe into the well and kill the well with heavy fluid to stop the fluid from erupting from the well. Embodiments of this invention are methods and apparatus to avoid failed risers and BOPs by constructing an alternative path to the subsea well that is not encumbered by the previously failed BOP, failed risers, or foreign debris lodged in the BOP
The current industry methods teach towards making the subsea BOP system reliable by stacking in a plurality of closure devices all in the same axis in a BOP stack, and to continually test the BOPs. However, if the BOP fails during a blow out, for example foreign object and debris like a large piece of earth or previously disposed casing or wellhead are pushed up into the BOP the BOP will not close and the foreign debris that is lodged in the BOP can also prevent the entry of fluid or drill pipe from surface to enter in the well to control the blowout. What is needed is an alternative path to the subsea well from surface that is unencumbered by the primary path. What is further needed is a method that presents an alternative path through drilling risers, and subsea BOPs and that allows a parallel subsea BOP stack and riser to be offset from the primary path and the first flow path axis of subsea BOPs and riser to a subsea well.
The current subsea industry is further challenged by the need to drill in ever deeper water depths and ever deeper subterranean depths below the subsea floor. A problem presents itself in deep water depths where the force that the sea depth places on the earth is less than the force that the overburden of the earth would places on subterranean formations. This results in the subterranean rock hydraulic fracture pressure of deep water offshore wells being lower than deep wells drilled from land. The drilling fluid hydrostatic pressure of the deep water wells weighted drilling fluid has a down hole hydrostatic pressure increased by the vertical height of the subsea wells seafloor depth to the surface of the sea. What is needed is a means to have the drilling mud from the sea floor, to the bottom of the well where the drill bit is cutting at a higher density, and the density of the drilling fluid between the outer diameter of the drill pipe and the internal diameter of the riser from the sea floor to the surface to have a lighter density. Embodiments of the invention allow for such a dual gradient drilling fluid means to be achieved by pumping through a second BOP conduit a lighter fluid and mixing it below the BOPs to create a lighter fluid hydrostatic from the seafloor to the surface in the drilling riser. This then allows wells to be drilled safer as the risk of lost circulation due to hydraulic fracturing of the subterranean rocks due to the combination of hydrostatic forces developed by heavy drilling fluids and drilling cuttings in the casing and open hole in addition to the hydrostatic forces of the drilling fluids and cuttings on the outer diameter of the drill pipe and the internal diameter of the drilling riser has been reduced.